1. Field of Invention
This invention relates to the surface and method for measuring a flow of bulk material. More specifically for surfaces for use in devices for metering (measuring) the mass flow ("bulk flow") of so-called bulk material. The invention also relates to designing a surface for use in impellers and propellers, and to methods of obtaining such surfaces.
2. Description of Prior Art
An example of the mass flow of bulk material is the flow of grain to the grain tank in a combine harvester. Surfaces in accordance with the invention are particularly suitable for use in flow meters that operate by measuring forces of this flow on a sensor surface. Surfaces in accordance with the invention may also be employed in e.g. hoppers, silos, harvesting and cutting machinery other than combine harvesters, conveying machinery and various kinds of manufacturing, and medical apparatuses.
Bulk flow may also embrace the flow of bulk grain and chemicals in transport vehicles (such as tankers, ships and railway tanker wagons); the flow of powders, and materials of larger particle size such as fruit, vegetables, coal, minerals and ores; and even the flow of liquids of high viscosity. Thus the invention may be of use in the measuring of characteristics of bulk flow in liquids whose viscosity changes. In general terms, bulk flow of material may in this context be regarded as any flow of matter in contact with a surface, in which the effects of friction between the surface and the material usually influence the maximum flow rate, and in which the matter exhibits free flow behaviour. Those skilled in the art will know that various parameters, such as the tendency of grains of bulk material to cohere and to adhere to the surface; and the effects of friction between individual grains, also influence the mass flow rate of bulk materials. However, the effects of such parameters are small compared with the effects of friction between the bulk material and the surface along which it travels.
The flow rate established by such flow a metering device as referred to above may be displayed continuously to an operator and may be used for establishing the total material flow over a time period. In a combine harvester, the operator may want to determine the amount of grain being collected in relation to the time being spent or the area being covered. Distinct mass flow measurement in combination with a measurement of the combine speed, the header width and the combine position can be used to establish the yield rate of the crop across the field being harvested. In this case, the mass flow meter has to be able to provide reliable yield data, not only over the total field, but also over relatively small field lots. The results may be used to create a yield map which the farmer can use for future crop treatment, such as targeted fertilizing and spraying of bad field lots.
Mass flow readings from prior art combine harvester mass flow measuring devices are known to be influenced by a variety of crop and harvest conditions, such that frequent recalibration of the flow meter is required in order to obtain reliable yield rate results under various circumstances.
For example, volumetric measurement devices as disclosed in European Patent application 0,042,245 and German application 3,045,728, meter the volume rate only. For the conversion to a mass rate the volume reading is multiplied by the specific mass value of the harvested crop. Under adverse crop conditions the yield rate will decrease. The grain kernel size may also be variable, such that the specific mass is affected and the measurement device needs readjustment.
Other mass flow meters, as shown in German application 2,947,414 and European application 0,208,025, comprise a straight or curved surface which is engaged by the mass flow. The resulting force in a predetermined direction is sensed by a sensor which generates a signal which is substantially proportional to the mass flow. In these systems the sensors are disposed along the bisector of the active portion of the surface.
These surfaces are subjected to the combination of centrifugal, gravity and friction forces of the layer of material flowing therealong. A decrease or increase of the friction coefficient .mu. between the bulk material and the metering surface will inevitably influence the material speed and the layer thickness along the surface and will consequently alter the centrifugal, gravity and friction forces. Thus the resulting total force in the direction of the bisector varies accordingly, so that recalibration of the flow meter may be required.
Such variations of the friction coefficient .mu. may occur in a harvesting machine when another crop variety is harvested, or when harvesting takes place under varying external conditions such as ambient temperature and humidity, field relief and orientation, neighbouring woodland, etc. A change in the settings of the harvester may result in the occurrence of more or less chaff in the grain sample, which will equally influence its friction characteristics and hence the measurement of the mass metering device.